1. Field of the Invention
The present invention relates to an optical disc device and a converging position correction method, and is suitably applied to, for example, an optical disc device.
2. Description of the Related Art
There has been widely used an optical disc device that is configured to reproduce information by irradiating an optical beam to an optical disc, such as a compact disc (CD), a digital versatile disc (DVD), and a Blu-ray disc (BD, a registered trademark), and reading reflected light of such an optical beam.
In addition, such an optical disc device records information by irradiating an optical beam to the optical disc and changing a local reflectivity and the like of the optical disc.
With respect to this optical disc, a size of an optical spot formed on the optical disc is obtained approximately by λ/NA (λ: a wavelength of an optical beam, NA: numerical aperture) and resolution is proportional to this value. For example, details of a BD which is an optical disc with a diameter of 120 mm capable of recording data of about 25 GB are shown in Y. Kasami, Y. Kuroda, K. Seo, O. Kawakubo, S. Takagawa, M. Ono, and M. Yamada, Jpn. J. Appl. Phys., 39, 756 (2000).
An optical disc records a variety of information, such as various contents including a music content and a video content, or various data for computer. In particular, in recent years, an amount of information has increased due to higher definition of video, higher sound quality of music, and the like, and increase in the number of contents to be recorded in one optical disc has been requested. Accordingly, the optical disc has been requested to have further increased capacity.
In view of the above, there has been suggested a method that increases recording capacity of one optical disc by superposing recording layers in one optical disc (for example, refer to I. Ichimura et al, Technical Digest of ISOM′ 04, pp 52, Oct. 11-15, 2005, Jeju Korea).
On the other hand, there also has been suggested an optical disc device using a hologram as a recording method of information on an optical disc (for example, refer to R. R. McLeod et al., “Microholographicmultilayer optical disc data storage,” Appl. Opt., Vol. 44, 2005, pp 3197).
For example, as shown in FIG. 1, an optical disc device 1 once converges an optical beam from an optical head 7 in an optical disc 8 made of photopolymer or the like in which a refractive index changes depending on optical intensity of irradiated light. Then, the optical disc device 1 uses a reflection device 9 provided on a back surface side (a lower side in FIG. 1) of the optical disc 8 to converge an optical beam on the same conversing position again from an opposite direction.
The optical disc device 1 emits an optical beam made up with laser light from a laser 2, modulates a light wave of the optical beam at an acousto-optical modulator 3, and converts the optical beam to parallel light by a collimator lens 4. Then, the optical beam passes through a polarization beam splitter 5, the optical beam linearly polarized is converted to be circularly polarized light at a quarter wavelength plate 6, and then is incident on the optical head 7.
The optical head 7 is configured to be able to record and reproduce information. The optical head 7 reflects the optical beam at a mirror 7A, converges the beam by using an objective lens 7B, and irradiates the beam to the optical disc 8 which is rotated by a spindle motor (not shown).
At this time, the optical beam is once focused in the inside of the optical disc 8, and then the optical beam is reflected by the reflection device 9 arranged on a back surface side of the optical disc 8, and is converged at the same converging point in the inside of the optical disc 8 from the back surface side of the optical disc 8. The reflection device 9 is configured with a converging lens 9A, a shutter 9B, a converging lens 9C, and a reflection mirror 9D.
At a result, as shown in FIG. 2A, a standing wave is generated at the converging position of the optical beam, and a recording mark RM made up with small holograms of an optical spot size is formed. The recording mark RM has an entire shape as though two cones are adhered to each other on the bottom surfaces thereof. In this manner, the recording mark RM is recorded as information.
The optical disc device 1 records a plurality of the recording marks RM in the inside of the optical disc 8 by rotating the optical disc 8 and arranging each of the recording marks RM concentrically or along a spiral track so as to form one mark recording layer. Further, the optical disc device 1 adjusts a converging position of the optical beam so as to be able to record each of the recording marks RM in a manner that a plurality of the mark recording layers are superposed.
In the above manner, the optical disc 8 has a multi-layer structure having a plurality of the mark recording layers in the inside. For example, as shown in FIG. 2B, in the optical disc 8, a distance (a mark pitch) p1 between the recording marks RM is 1.5 μm, a distance (a track pitch) p2 between tracks is 2 μm, and a distance p3 between layers is 22.5 μm.
In addition, in case information is reproduced from the disc 8 on which the recording mark RM is recorded, the optical disc device 1 closes the shutter 9B of the reflection device 9 so as not to radiate the optical beam from the back surface side of the optical disc 8.
At this time, the optical disc device 1 irradiates an optical beam to the recording mark RM in the optical disc 8 by using the optical head 7, and emits a reproduction optical beam generated from the recording mark RM to the optical head 7. The reproduction optical beam which is circularly polarized is converted to be linearly polarized at the quarter wavelength plate 6, and reflected by the polarization beam splitter 5. Further, the reproduction optical beam is converged by a converging lens 10, and irradiated to a photodetector 12 through a pinhole 11.
At this time, the optical disc device 1 detects a light amount of the reproduction optical beam at the photodetector 12, and reproduces information based on a result of the detection.
In addition, there has been suggested an optical disc device that uses different kinds of optical beams between position control of an objective lens and recording and reproduction of information (for example, refer to S-K Park, T. D. Milster, T. M. Miller, J. Buts and W. Bletscher, Jpn. J. Appl. Phys., Vol. 44 (2005) pp. 3442-3444).
For example, as shown in FIG. 3, an optical disc device 15 irradiates a position control optical beam L1 to an optical disc 18 through a beam splitter 16 and an objective lens 17.
In addition, the optical disc device 15 detects return light obtained by the position control optical beam L1 reflected on a reflection surface 18A of the optical disc 18, carries out position control, such as focus control and tracking control of the objective lens 17 in accordance with a result of the detection, and focuses the position control optical beam L1 at a desired track of the reflection surface 18A.
In this state, the optical disc device 15 reflects a recording and reproduction optical beam L2 which is different from the position control optical beam L1 by using the beam splitter 16, focuses the optical beam L2 on a recording layer 18B of the optical disc 18 through the objective lens 17 which is position-controlled, and in this manner carries out recording or reproduction of information (the recording mark RM and the like).